Self-compensating tachometer generator

ABSTRACT

A tachometer generator has rotor and stator pole pieces which interfit as they move relative to one another so as to provide at least a pair of relatively low reluctance stator-rotor flux paths parallel to the axis of rotation of the rotor.

United States Patent Inventor Dan Goor Lexington, Mass.

Appl. No. 848,465

Filed Aug. 8, 1969 Patented Feb. 16, 1971 Assignee Trans-Sonics, Inc.

Lexington, Mass.

SELF-COMPENSATING TACHOMETER GENERATOR 2 Claims, 4 Drawing Figs.

U.S. CI. 310/168 Int. Cl. 110211 19/24 Field 01' Search 3 10/46,

[56] References Cited UNITED STATES PATENTS 1,892,371 12/1932 Tuczek 178/6 2,913,662 11/1959 Hogan 324/70 3,161,803 12/1964 Knittweis.. 310/168 3,230,407 1/1966 Marsh Primary Examiner-Milton O. Hirshfield Assistant Examiner-R. Skudy Attorney-Blair Cesari and McKenna ABSTRACT: A tachometer generator has rotor and stator pole pieces which interfit as they move relative to one another so as to provide at least a pair of relatively low reluctance stator-rotor flux paths parallel to the axis of rotation of the rotor.

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26d I'- 34c -42b 24 7 42a 34 SIGNAL CONDITIONING UNIT INVENTOR DAN 600R 15m mwww ATTORNEYS SELF-COMPENSATING TACHOMETER GENERATOR BACKGROUND OF THE INVENTION l.Field of the Invention Y This invention relates to a tachometer generator. lt relates more particularly to a tachometer generator whose output remains relatively unaffected by vibration, shock and the fact that the generator parts and installation tolerances may be relatively loose. I r I A tachometer is used to measure the speed of rotation of a shaft. The type to which this invention is directed 'co'mprises an electrical generator whose rotor is mechanically coupled to the shaft and an output device that converts the signal from the generator to a directly usable form. In one prevalent application, such tachometers are used to indicate the speeds, axles or drive shafts in automobiles. The tachometers of primary concern here are particularly adapted to be parts of electronic antiskid brake controls for automobiles. As such,the present tachometer generator may beused in the tachometer disclosed in US. application Ser. No. 835,133, filed, June 20, Y

1969, entitled Tachometer Generator, which application is owned by the assignee of the present application.

2. Description of the Prior Art v The generator in a tachometer is often similar to a conventional electrical generator. An AC generator is commonly used to order to eliminate the brushes needed in DC generators. Also, this permits the use of the generator output frequently as the speed-indicating parameter. The frequency is absolutely proportional to speed and a more accurate speed indication than the output amplitude; which is not so linearly related to speed.

In many applications, the use of generator frequency largely avoids the effects of various factors that affect the amplitude of the generator signal. These factors include radius variations of the rotorand stator, eccentricity of these parts and other variations encountered in machining'and assembling the various parts. However, where low costis an important factor, manufactur ng tolerances have to be relaxed, with a consequent increase iri amplitude variations. These variations may then be too great for the devices using the generator output to cope with.

This problemis further complicated in automobile brakecontrol systems where, because of the nature of automotive assembly, the generator rotor and stator cannot be installed as a preassembled unit. in the early state of assembly, the rotor is affixed to the shaft (or axle) whose speed is to be monitored; and the stator is fixed in place after the shaft has been assembled with its housing. understandably, this can result in substantial misalignment or rotor and stator, with a great deal of variation in generator output. Further changes may stem from the shock and vibration normally encountered by the undercarriage of an automobile.

The above noted application Ser. No. 835,l33 discloses a tachometer generator whose rotor is a toothed, ferromagnetic ring mounted on the shaft or axle whose speed is to be monitored. The stator has a single pole positioned adjacent the ring, a magnet arranged to pass flux through the pole and into the rotor, and also a coil that develops a voltage in response to changes in the stator flux. Movement of the rotor teeth past the stator pole changes the stator flux, causing the coil to develop a signal whose frequency is proportional to the rotor speed. While this generator is well suited for the automotive environment and. indeed, is better adapted for use in brakecontrol systems than prior tachometer generators used in such systems, further improvement is desirable.

SUMMARY or THE INVENTION More specifically, it is an object of the present invention to provide a tachometer generator having relatively wide manufacturing tolerances, yet which develops a relatively strong output signal.

Another object of the invention is to provide a tachometer generator which develops a relatively strong outputsignal, even through it is subject to shock and vibration in use.

LII

Another object of the invention is toprovide an accurate tachometer generator, having the foregoing characteristics, which is relatively easy to install and whose rotor and stator may be separately installed without unduly complicating normal automotive assembly procedures.

Other objects will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth and the scope of the invention will be indicated in the claims.

in its preferred form, the present tachometer generator is similar to the one described in the above-mentioned patent application, although the principles ofthe invention are also applicable to other generator designs. Accordingly, the rotor is preferably made of a magnet material and has a circumferential array of poles or teeth. A stator, having 'a single pole,

is positioned adjacent the periphery'of the rotor so that when the rotor turns, successive teeth thereon move past the end of the stator. This varies the flux between the stator pole piece and the rotor and a coil on the stator develops an output voltage in response to these flux changes, the frequency of this signalbeing proportional to the speed of the shaft or axle on which the'rotor is mounted. Y r

My improvement resides in a unique interfitting of rotor and stator poles which makes the generator output relatively immune to variations in the relative positions of the rotor and stator. y

More particularly, in one embodiment of the invention, the end of the stator pole piece facingthe rotor is provided with a generally U-shaped notch so that as the rotor turns, its teeth move insuccession between the legs of the notch. As each rotor tooth passes through the U-shaped notch, there is a low reluctance path between the tooth and the stator pole piece not only at the end of the tooth'but also at the sides thereof. Consequently, substantial magnetic flux passes between the stator and the rotor. This means that as the rotor teeth relative to the stator, there is a relatively large flux change and a correspondingly large voltage induced in the stator coil.

Moreover, this construction permits relaxation of the parts tolerances and assembly standards without seriously weakening the output signal from the generator. For example, even if some of the rotor teeth are shorter than others, a relatively strong magnetic flux passes between the legs of the' stator pole piecenotch and the sides of these teeth. Consequently, as these shorter teeth move through the stator, there is still sufficient flux change to enable the stator coil to generate a relatively strong output voltage. The same is true if the rotor happens to be slightly eccentric so that the teeth on one side thereof are brought closer to the stator pole piece than those on the other side.

The present generator construction also makes the generator output signal relatively immune to changes in alignment between the rotor and stator pole pieces. These changes may stem from the fact that the rotor is slightly cocked on its supporting shaft or that the stator pole piece is displaced axially relative to a centered position opposite the rotor. These variations may be due to vibration, shock or lax installation procedures. This accommodation of misalignment is due to the fact that any relative sidewise movement between the rotor pole pieces and the stator pole necessarily moves the rotor teeth farther from one leg of the stator notch, but at the same time, closer to the other leg of the notch. Consequently, the total magnetic flux is not unduly changed over a relatively wide range of misalignment.

Thus, the present tachometer generator provides a fairly large amplitude output signal to which other electronic devices in a brake control system can easily respond. Yet, the manufacturing tolerances of the generator components are not too critical and the generator is more easily installed in an automobile because the rotor and stator components do not 'have to be in perfect alignment.

BRIEF DESCRIPTION OFTI-IE DRAWINGS For afuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which: a i

FIG. 1 is a fragmentary side elevational view, with parts in section, of a tachometer generator embodying the principles DESCRIPTION OF THE PREFERRED EMBODIMENT.

Referring now to FIGS. land2 of thedrawing,a tachometer indicated generally at 8 is used to sensethe speed of 'a rotary shaft 12. Thetachometer includes a generator 10 comprising an annular rotor l4 affixed to the shaft 12, and a stator 16 closely spaced from the periphery of rotor 14. When shaft 10 rotates, stator 16 develops electrical signals which-are applied by way of a signal conditioning unit 118 to a responsive device 20 which maycomprise control circuitry for an antiskid brake systemor simply, an indicating meter. The conditioning unit 18 converts the output of stator 16 to a form which is particularly suited to drive the responsive device 20.-

As best seen inFIG. l, rotor 14 consists ofa ring 22 titted'to rotate with shaft '12 and having a peripheral array of radial teeth 24. Ring 22 and teeth 24 are made-of a'material having a .relatively high magnetic permeability material.

Stator'lfi comprises a core-26 whose end 26a opposite rotor 14 forms a specially shap'ed pole piece to be described in detail later. A permanent magnet 28 is positionedat the opposite end of core 26 and a coil 30 around the'core develops a voltage in response to flux changes in the core. The ends of the coil are electrically connected to signal-conditioning unit 18.

The flux from magnet 28 passes through core 26 and thence to rotor 14 by way of the intervening air gap 34. The length of air gap 34 is substantially less when a rotor tooth 24 is opposite the polepiece 26a'than when an intervening airspace 36 is opposite the pole piece. Consequently, the reluctance of air gap 34 decreases and increases as successive teeth 24 and gaps 34 proceed past pole piece 26a during rotation of the shaft 12. The flux through core 26 increases and decreases with these changes in reluctance, thereby inducing a voltage in coil 30. The frequency of this signal is directly proportional to the rotational speed of shaft 12.

Referring now to FIG. 3, the rotor and stator pole pieces 24 and 26a, respectively, coact to minimize the effects of inaccuracies in the rotor and stator components themselves or in theirplacement relative to one another. More particularly, the

rotor and stator pole pieces are shaped to inter-fit with one anotheras the rotor turns. The stator pole piece 26a is formed with a generally rectangular recess 40 in its end face.'Recess 40 is shaped and positioned so that when shaft 12 rotates, each rotor pole piece or tooth 24 projects appreciably into recess 40. In other words, the pole piece legs 42a and 42b defining channel 40 extend down and overlap each tooth 24 at the opposite sides thereof as the tooth-swings by the stator. I

This interfitting stator and rotor pole piece construction provides a flux path not only in the air gap portion -34a between the-top of each tooth 24 and the bottom of channel 40, but also in the portions 34b and 340 between the opposite sides of tooth 24 andpole piece. legs 42a and 42b. Consequently, when each tooth' 24 is positioned in the recess 40 in FIG. 3, there-is materially greater stator-rotor flux than in prior pole piece arrangements; this results in an increased difference between the maximum flux and the minumum flux which obtains when a rotor air space 36 is disposed opposite the stator pole piece. Accordingly, there is a desirable increase in the output voltage, which corresponds to the change It will be seen from the foregoing in magnetic flux'as successive rotor teeth 24 and spaces 36 pass the stator pole piece 260. Y

Moreover, even if the radial length of gap portion 340 should be unusually large due to an eccentricity in shafl 12 or rotor 14, or an unusually short tooth 24 length, or improper positioning of the stator 16, there is still a larger gap in the side portions 34b and 340 of the gap. Therefore, there is still sufficient magnetic flux change to induce a relatively strong and easily sensed voltagepulse in coil 30.

In addition, even if the rotor and stator pole-pieces are out of alignment or cocked due to faultyinstallation of rotor 14 or stator 16, there isalways a short air gap portion between at least some parts of the rotorand stator pole pieces. That is, if rotor 14 in FIG. 3 is cocked clock'wise,the radial length of a part of air gap portion 34a increases-and the length of portion =34c increases. However, this is compensated by a decrease in the length of gap portion 34b. Similarly, if the stator 16 is displaced in the axial direction, the reluctance of one of the gap portions 34b and 34cincreases, but this islargely compensated by a'decrease in-the r'eluctancein the'other gap portion.

" It will be obvious also that the output of the present tachometer generator is relatively unaffected by vibrations and shocks encountered by the generator-in'ordinary use. That is, most suchvibrations'affect the radial widthof air gap 44 or cause misalignment or cocking of the rotorv and stator pole-pieces. However, the interfitting pole piece construction described above compensates to a large degree for these irregularities; in other'words the invention decreases the dependence of output voltage and the relative positions of the rotor -l"4andstatorl6. 1' Y then that my improved tachometer generator pole piece construction allows wider latitude in the setting of parts and ass'ernblytolerances.This,

in turn, lowers the manufacturing and installation-costs of the present system. Yet, at the same time, the present system responds very quickly to changes in shaft speed, even at relatively low shaft speeds.

While we have shown an interfitting rotor and stator pole receive the rotor pole piece, onecan produce substantially the same effect by notching the rotor teeth 24 to, receive the stator Pole piece 26a.

FIG. 4 illustrates diagrammatically another generator embodiment whose stator 52 has three pole pieces, 54a, 54b and 540, defining the recess 40. The pole piece 540 is opposite the ends of the rotor teeth 24 and the pole-pieces 54b and 540 are opposite the sides of the teeth. Three series connected windings 56a, 56b and 56c are disposed on the respective pole pieces. Thus, the output voltage of the generatoris the sum of the voltages induced in the windings 56.

The FIG..4 generator functions in'much the same manner as the FIGS. l3 embodiment. In particular, the sum of the winding voltage varies, in proportion, much less than the voltage in any individual winding if the rotor and stator are cocked or displaced relative to one another, or the rotor is irregular or eccentric.

It should be noted that the above description relates specifically to a generator whose stator has a single pole, and is applicable to multipole stators. In general, the invention i's'more readily applied to stators that extend only part way around the rotor than to those that completely encircle it.

It wili' thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understoodthat the following claims-are intended to cover all of the generic andspecific features of the invention herein described. 1

I claim:

1. A tachometer generator comprising:

A. a rotor:

I. made of material having a high magnetic permeability; flux paths generally parallel to the axis so that relatively 2. mounted for rotation about an axis; and 7 large changes in the rotor-stator flux from said magnet 3. a plurality of pole pieces arranged about said axis; and occur in response to rotation of the rotor; and

B. a stator including:

1. a permanent magnet; 2. a stator pole piece positionedopposite the rotor radi- W Ward from the axis b either h rotor pole 2. A tachometer generator as-defined in claim I wherein pieces or the stator pole piece having radial passages for receiving the opposite member so that the rotor and each stator pole piece has a notch at its end through which the stator pole pieces interfit as they move relative to one 10 rotor Pole p'eces pass as h rotor another, thereby providing relatively short rotor-stator 3. a winding associated with the stator pole piece and arranged to develop a voltage in response to said flux changes. 

1. A tachometer generator comprising: A. a rotor:
 1. made of material having a high magnetic permeability;
 2. mounted for rotation about an axis; and
 3. a plurality of pole pieces arranged about said axis; and B. a stator including:
 1. a permanent magnet;
 2. a stator pole piece positioned opposite the rotor radially outward from the axis thereof, either the rotor pole pieces or the stator pole piece having radial passages for receiving the opposite member so that the rotor and stator pole pieces interfit as they move relative to one another, thereby providing relatively short rotor-stator flux paths generally parallel to the axis so that relatively large changes in the rotor-stator flux from said magnet occur in response to rotation of the rotor; and
 3. a winding associated with the stator pole piece and arranged to develop a voltage in response to said flux changes.
 2. mounted for rotation about an axis; and
 2. a stator pole piece positioned opposite the rotor radially outward from the axis thereof, either the rotor pole pieces or the stator pole piece having radial passages for receiving the opposite member so that the rotor and stator pole pieces interfit as they move relative to one another, thereby providing relatively short rotor-stator flux paths generally parallel to the axis so that relatively large changes in the rotor-stator flux from said magnet occur in response to rotation of the rotor; and
 2. A tachometer generator as defined in claim 1 wherein each stator pole piece has a notch at its end through which the rotor pole pieces pass as the rotor turns.
 3. a winding associated with the stator pole piece and arranged to develop a voltage in response to said flux changes.
 3. a plurality of pole pieces arranged about said axis; and B. a stator including: 